1. Field of the Invention
The invention relates to methods and apparatus for cleaning articles using supercritical and/or near-supercritical fluids. In particular, the present invention relates to using differences in contaminant solubility and solvent density at various temperatures and/or pressures to effect cleaning action, to influence solvent and/or contaminant movement in the cleaning apparatus, and to facilitate the concentration of contaminants within the cleaning apparatus and their subsequent removal. Even more particularly, the invention relates to an apparatus comprising multiple heating, cooling, and/or cleaning zones, which result in a jet flow of solvent/cleaning fluid onto the article to be cleaned without the need for a pump or compressor. The jet flow provides more effective contaminant removal, and the multiple zones also result in increased residence time for increased efficiency in separating the contaminant from the solvent/cleaning fluid.
2. Background of the Invention
Solvents commonly are used to remove organic and inorganic contaminants from articles. The contaminated article to be cleaned is contacted with the solvent to solubilize and remove the contaminant. In a vapor degreaser, subsequent evaporation of the solvent separates the solvent and the contaminant, and the solvent vapors are redirected to the article to further clean it. The contaminant typically is concentrated in the evaporation step, removed as a precipitate, as a separate liquid phase, or as a concentrated solution in the original solvent.
Grease may be removed from the surface of metal castings and other nonabsorbent bodies by means of solvents, while contaminants collect in the bottom of the apparatus and are drawn off from time to time through a valve. One of the drawbacks of this type of cleaning process is that the cooling surfaces have a tendency to also condense water out of the atmosphere in addition to cooling and condensing the solvent. This condensed water then becomes associated with the solvent and comes into contact with the metal parts of the cleaning apparatus and with the article being cleaned.
The problem of condensed water with the solvent may be overcome by first contacting the atmosphere with condensing surfaces at a temperature above the dew point of the atmosphere in which the operation is being carried out, but substantially below the condensing temperature of the solvent. The condensed solvent is drawn off for use in the cleaning process, while the remaining vapors are brought into contact with still cooler surfaces (cooler than the dew point) to condense out the water so it can be removed. Alternatively, the article itself may be cooled. Vapors of a solvent may be generated from a liquid sump and a desired level of solvent vapor established by adjusting the temperature of the condenser. A contaminated cold article is introduced into the solvent vapors, thereby causing the vapor to condense on the article. Condensate containing the contaminant falls from the article into the sump, and the article is removed from the solvent vapor when its temperature reaches the solvent vapor temperature (thus precluding further solvent condensation on the article).
Cleaning Using Supercritical Fluids
In an effort to improve on vapor degreasing methods, supercritical (and near-supercritical) fluids have been used as solvents to clean contaminants from articles. NASA Tech Brief MFS-29611 (December 1990), describes the use of supercritical CO2 as an alternative for hydrocarbon solvents conventionally used for washing organic and inorganic contaminants from the surface of metal parts.
In a typical supercritical fluid cleaning process, the part to be cleaned is contacted with a supercritical fluid. The supercritical fluid, containing solubilized contaminants removed them from the part, flows to a zone of lower pressure through an expansion valve. The resulting depressurization causes the state of the solvent fluid to change from supercritical to subcritical, resulting in separation of the solute (that is, the contaminant) from the solvent. Relieved of its burden of contaminant, the cleaned solvent fluid is compressed back to a supercritical state and again brought into contact with the part if further cleaning is desired.
Alternately, the article to be cleaned, such as a silicon wafer is placed in an atmosphere of supercritical carbon dioxide which contacts the wafer to solubilize the contaminant. After cleaning is complete, carbon dioxide is cooled to below its supercritical temperature (i.e. the system pressure is reduced and the carbon dioxide attains equilibrium between the liquid and gas phases) before removal of the cleaned wafer from the apparatus.
While effective, the foregoing processes are relatively inefficient because of the energy consumed in each pressurization-depressurization cycle. Further energy losses and increases in equipment complexity are associated with moving the solvent through the apparatus in both supercritical and subcritical states.
An apparatus for removing contaminants from an article to be cleaned in a pressure vessel comprising;
a first zone and a second zone separated by a first thermally insulated baffle, said first zone comprising at least a first heating element adapted to direct a fluid from said first zone to a second zone;
a third zone separated from said second zone by a second thermally insulated baffle, said second zone comprising at least a second heating element adapted to direct said fluid from said second zone to said third zone;
said third zone being separated from a fourth zone by a third thermally insulated baffle, said third zone being adapted to retain said article to be cleaned and comprising at least a third heating element adapted to direct said fluid from said third zone to said fourth zone, said third zone further comprising at least one cooling element and at least a first static baffle adapted to divert at least a portion of said fluid being directed from said fourth zone onto said article to be cleaned, producing a natural convective fluid flow at a rate effective to remove contaminants from said article to be cleaned.